The invention relates generally to a method and apparatus for recognizing an electrical power failure and, in particular, to a method and apparatus for detecting either a current or a voltage loss, which creates an electrical condition under which electrical circuitry might suffer irreversible adverse consequences if suitable countermeasures were not otherwise taken.
The detection of an AC power failure or outage is, for many classes of electrical equipment, an important and necessary function to prevent damage to the electrical components and to prevent loss of information stored by many electrical storage elements. Conventional apparatus have employed, for example, a low pass averaging circuit which, when the output decreases below a threshold level, triggers a failure response. This circuitry, while simple, has as its principal disadvantage substantial delay before the circuitry responds to an outage. In more recent applications, response time improvements have been achieved by the insertion of a frequency multiplier stage before the low pass filter. This permits the low pass filter to have a shorter time constant so that the delay between the power outage and the recognition thereof decreases.
Another conventional apparatus employs zero-crossing prediction which reduces the maximum delay to at most one-half cycle at the power frequency. Such a circuit, however, is insensitive to the voltage amplitude so that conditions of severe voltage reduction may not be detected even though the electrical equipment becomes dysfunctional.
Another method of recognizing a power failure utilizes a phase-locked-loop synchronized to the AC power frequency. A power outage is recognized if the loop drops out of a "lock" condition. This method introduces a detection delay due to the time constant of the low pass filter in the oscillator feedback loop. The filter is necessary for the circuit to be insensitive to random voltage transients.
There exist current measuring, power failure detection methods and apparatus. The measurement of current, however, is often associated with the undesirable insertion into a line of a relatively high loss circuit element, such as a resistor or plural diodes, across which a voltage measurement is made.
It is therefore an object of the present invention to detect a power outage including a "brown out" condition wherein there are severe voltage reductions. Other objects of the invention are a method and apparatus providing rapid response to a power outage, and a method and apparatus which are reliable and low in manufacturing cost. Further objects of the invention are a method and apparatus for measuring current flow which is reliable, low in cost, and has a low insertion loss. A further object of the invention is a method and apparatus which performs this detection while remaining safely isolated from hazardous electrical potentials on the conductor being monitored.